Four distinct
epitopes (A, B1, B2, and C) have been functionally defined on the human
alpha4 integrin. In this study, two cross-reactive antihuman alpha4
monoclonal antibodies (mAb) (HP2/1 and HP2/4 specific for
epitopes B1 and B2, respectively) were used to functionally characterize the rat
VLA-4 subunit and to define similar functional
epitopes in this rodent species. It was found that B1 and B2 anti-alpha4 mAb completely block adhesion to
fibronectin, but the inhibition of adhesion to
vascular cell adhesion molecule-1 (VCAM-1) with HP2/1 mAb was lower than with HP2/4 mAb. It was also observed that
epitope B2 HP2/4 mAb induced homotypic aggregation in rat lymphocytes, whereas
epitope B1 HP2/1 mAb did not. Using the
HgCl2 model of
nephritis, this study shows the protective effect of both anti-alpha4 mAb against infiltration of the renal interstitium by leukocytes. Nevertheless, HP2/1 mAb, but not HP2/4 mAb, virtually abolished the anti-glomerular basement membrane antibody synthesis and glomerular deposits. These findings indicate the dual but independent role played by alpha4
integrins in both extravasation of leukocytes and in the production of
antibodies. Finally, this study demonstrates that anti-rat
VCAM-1 mAb showed a positive reactivity of the renal vascular endothelium and, most importantly, that administration of anti-VCAM-1
antibodies completely abrogated the interstitial cell infiltrates without affecting anti-glomerular basement membrane antibody production. These results confirm the important role played by VLA-4/VCAM-1 pathway in leukocyte infiltration, and further support the dual and independent role of alpha4
integrins in both renal infiltration and
autoantibody synthesis in this model of renal disease.